We propose to continue experiments on the synthesis of ATP from ADP and inorganic phosphate by (Na plus K ions)-ATPase prepared from guinea pig kidney membranes. We will try to determine if the determining factor is the binding of Na ion to the enzyme or the translocation of Na ion from one side to the other of the membranes. We propose to isolate from homogenates of guinea pig kidney, or other tissues, vesicular preparations of (Na plus K ions)-ATPase which are homogeneous with respect to sidedness and leakiness so that we can study phosphorylation reactions of (Na plus K ions)-ATPase with knowledge of which side of the membrane the various ions are acting on. We propose to complete experiments demonstrating that the form of the native phosphoenzyme which equilibrates with ATP is a precursor of the form which equilibrates with inorganic phosphate in the normal forward reaction sequence.